Short Buyin

1. A buyin that is smaller than the minimum or standard amount for a given table.

B&M

In a brick and mortar poker setting (and sometimes online too), a short buyin is usually allowed only as a one-time courtesy.

So if the minimum buyin in a $10/20 NL game is $600, a player might be allowed to buy in for $400 just one time if he busts; the next time he busts, he will have to go to the ATM in order to pony up the full $600 amount.